The present invention is directed to a method and apparatus in a communication device, such as a selective call receiver, and more particularly to a communication device capable of eliminating a symbol phase and frequency difference between transmitted symbols and apriori symbols generated within a correlator.
The Maximum Likelihood Detector which is also known as the Optimum Noncoherent Detector (or correlation detector) for detecting frequency shifted keyed (FSK) signals in an additive white gaussian noise channel is well known. The performance of a correlation detector can also be achieved with other detector architectures such as a Matched Filter or a Fast Fourier Transform (FFT). However, the ability of these detectors to achieve a significant sensitivity improvement (i.e., 4 dB) over that of discriminators for M-level orthogonal signaling (i.e. FLEX(copyright)) or up to 3 dB improvement for 4-level quasi-orthogonal signaling (i.e., ReFLEX(copyright)) depends greatly on the frequency offset between the transmitter and receiver. For example, acceptable performance of the correlation detector (in a FLEX receiver) requires the transmitter""s carrier frequency and receiver""s local oscillator frequency to match better than about 0.2 parts per million (ppm) at 900 MegaHertz. Stable frequency references accurate to this level of precision are not available at a reasonable cost. At least 5 to 10 ppm of residual frequency offset error is typical in today""s radios. Thus, what is needed is a correlation detector having an automatic frequency control (AFC) system capable of providing acceptable performance in terms of eliminating frequency offset error at very low cost.